A shear injury is a type of traumatic brain injury (TBI) usually caused by rotational forces that damage the connections between the white and gray matter. So, what are the causes of shear injuries, and how do you identify shear trauma on a brain computed tomography (CT) scan?
Shearing occurs when forces are directed in opposite directions. A simple example is when you tear paper with your hands. Each hand pulls the paper in opposite directions, resulting in a separation (e.g., tearing) of the paper.
Shearing can occur in complex patterns in both air and in liquids as well. One easy demonstration you can try is using shear force to tell if an egg is hard boiled.
If you spin a fresh egg on a table, and then gently rest your finger on it to stop it from spinning, it keeps spinning a bit because the liquid contents continue to rotate after the shell stops. A hard-boiled egg, on the other hand, will stop completely because the inside is solid. In the case of the fresh egg, shearing forces occur at the interface between the eggshell and the moving inner contents of the egg, which continue briefly after you stop its rotation.
How do rotational forces cause damage during a TBI?
During a TBI, rotational forces are often experienced by the brain. When this happens, shear forces typically occur at the interface between gray and white matter in the cortex, presumably because of differences in their structures.
Shear forces can also be the result of movement of the brain inside the skull, like the egg contents in the shell. Trauma-induced shear forces frequently result in structural damages in the brain. This is because the brain is among the softest organs in the body and behaves more like gelatin than a rigid solid!
There is some speculation that the woodpecker does not experience brain injury during its avid pecking because the direction of impact of its beak with the tree is linear, thus minimizing the rotational forces on the brain.
Identifying shear hemorrhages on a CT scan
The effect of shearing along the interface between gray and white matter can result in small hemorrhages, which are most evident on susceptibility weighted magnetic resonance imaging (SWI), a magnetic resonance imaging (MRI) technique used to detect blood products.
While shear hemorrhages are not as evident on CT as they are on MRI, you should look for any small parenchymal hemorrhages both at the level of the midbrain and the junction of the gray and white matter. These hemorrhages are typically the result of shearing or distraction forces at the time of a traumatic brain injury (TBI), and while only one or two are visible, the injuries may be more extensive than is apparent on CT or MRI.
Always consider shear hemorrhages when you see high attenuation at the gray-white matter junction, the corpus callosum, or the midbrain in a trauma patient.
High attenuation may be the tip of the iceberg with regard to overall brain injury. This is because the shearing forces may cause other non-hemorrhagic axonal injuries in the brain. When widespread, it can result in what is called diffuse axonal injury or DAI.
Identifying diffuse axonal injury (DAI) on a brain CT scan
At one time, DAI was an autopsy finding that indicated a widespread disruption of connecting fibers in the white matter as the result of trauma. Many of those patients with DAI never regained consciousness.
You may see the term DAI used on CT reports when there are appropriate symptoms and small hemorrhages at the gray-white matter interface.
Clinical case: multiple shear injuries but minimal symptoms
A brain CT scan of a college basketball player who fell on the wooden court floor during a layup demonstrated a skull base fracture and a focal area of high attenuation at the gray-white junction (Fig. 7). He was briefly unconscious and disoriented after the fall, but he recovered completely soon afterwards except for persistent headaches. At that point, an MRI was ordered for him as an outpatient.
The SWI from his MRI demonstrated multiple punctate hemorrhages in the left frontal lobe, many more than were seen on CT. At one time this might have been considered a sign of DAI, but this patient was neurologically normal and had returned to school.
The sensitivity of MRI has made small shear injuries much more apparent, so they can be detected in patients without significant neurological deficits after trauma. So, be mindful that not all patients with multiple shear hemorrhages have symptoms of DAI.
That’s it for now. If you want to improve your understanding of key concepts in medicine, and improve your clinical skills, make sure to register for a free trial account, which will give you access to free videos and downloads. We’ll help you make the right decisions for yourself and your patients.